1. Field of the Invention
The present invention relates to a dust core used for a magnetic core of a transformer or an inductor, a magnetic core for a motor or other electronic parts.
2. Prior Art
In recent years, progress in miniaturizing electric or electronic tools has been made. Along with such a progress, there is a demand for small-sized and highly efficient dust cores. As a ferromagnetic powder for a dust core powder, a ferrite powder or a ferromagnetic metal powder is used. Because the ferromagnetic metal powder has a large saturation magnetic flux density in contrast to the ferrite powder, it has the advantage that the magnetic core can be small-sized. However, the ferromagnetic metal powder has a low electric resistance. Thus, it has the drawback that the eddy-current loss is increased. A dielectric film is formed on the surface of a ferromagnetic metal powder with an insulating material such as a resin or an inorganic material to decrease the eddy-current loss as much as possible.
Other than the above, the characteristics required to miniaturize a magnetic core include not only a large saturation magnetic flux density but also a high magnetic permeability (effective magnetic permeability in an applied field) in a high magnetic field of superimposed direct current to alternating current. Excellent direct current superimposition characteristics enables the miniaturization of the magnetic core. This reason is as follows. The strength of an operating magnetic field is obtained by dividing a current by the length of a magnetic path. Therefore, when the magnetic core is small-sized whereby the length of a magnetic path is shortened, the operating magnetic field is transferred to the high magnetic field side. Even if the operating magnetic field is transferred to the high magnetic field side, a high inductance is obtained, enabling miniaturization, if the magnetic permeability when direct current is superimposed is high.
Also, other than the above, an inductor corresponding to a large current is required. In this case, also, even if the current is increased and the operating magnetic field is transferred to the high magnetic field side, this can be dealt with when the magnetic core has a high magnetic permeability in a high magnetic field. Further, if the magnetic core has a high magnetic permeability in a high magnetic field and is free from a sudden reduction in magnetic permeability, the number of windings in, for example, an inductor can be increased. Because the inductance of an inductor is proportional to the square of the number of windings, the magnetic core can be smaller.
On the other hand, even if the magnetic core has a high magnetic permeability in a high magnetic field, core loss comes to be important along with the progress in the miniaturization of the magnetic core. Conventionally, when a ferromagnetic metal powder is molded to prepare a dust core, it is heat-treated at high temperatures to improve the magnetic characteristics such as core loss, thereby releasing the strain caused by molding to decrease the coercive force of the dust core to thereby improve the direct current superimposition characteristics. Also, hysteresis loss is decreased and in addition, core loss can be decreased.
However, high temperature heat treatment like this causes a resin in an insulating material to decompose rendering its amount reduced thereby decreasing electric insulation between ferromagnetic metal powders. This causes the eddy-current loss to be increased and hence the core loss is increased.
In view of the above situation, the following proposals have been offered to prevent the core loss from increasing. For instance, dust cores and the like using a silicone resin as an insulating material are disclosed in each of the publications of JP-A-2000-49008, JP-A-2000-30925, JP-A-2000-30924, JP-A-11(1999)-260618, JP-A-8(1996)-236333, JP-A-7(1995)-211532, JP-A-7(1995)-21153 and JP-A-6(1994)-342714. Also, a dust core and the like which use a silicone resin and an organic titanate as an insulating material are disclosed in the publications of JP-A-8(1996)-45724 and JP-A-7(1995)-254522.
However, the silicone resin used in such a dust core and the like described in the publication of JP-A-2000-49008 as aforementioned poses the problem that if the heat-treating temperature is raised, the silicone resin is heat-decomposed rendering its amount reduced thereby decreasing electric insulation between ferromagnetic metal particles, which causes the eddy-current loss to be increased and hence the core loss is increased.
Further, the reduction in the amount of the silicone resin as a result of the heat-decomposition of the silicone resin likewise poses the problem of reduced mechanical strength because of a reduction in the amount of the binder between ferromagnetic powder.
Therefore, it is an object of the present invention to provide a dust core which has a high magnetic permeability representing the direct current superimposition characteristics, which has reduced core loss and which has increased mechanical strength even if it is heat-treated at high temperatures, the dust core being obtained by pressure-molding at least a ferromagnetic powder and an insulating material.